The present invention is concerned with the field of photon sources and methods for fabricating photon sources. In particular, the present invention is concerned with photon sources which may be configured as single photon sources whose output may be controlled to the accuracy of a single photon.
Recently, there has been considerable interest in developing photon sources which can be configured as single photon sources for use in quantum cryptography for distributing a quantum key and other quantum information processing systems. Further, potential applications also exist in low-noise and low-signal metrology.
A photon source, which may be used for these types of application, has previously been described in GB 2 367 690. This patent application describes both optically excited single photon sources and electrically excited single photon sources. Although the initial results for these types of photon sources are extremely positive, in order to develop a source optimised for use with standard telecom optical fibres, the emission wavelength should lie close to 1.3 μm or 1.55 μm.
Quantum dots which emit at the required energy or wavelength can be fabricated from InAs or InGaAs using a self assembled growth technique. Here, the quantum dot layer is formed on an underlying layer which has a mismatched lattice constant to that of the material of the quantum dots when unstrained.
WO01/93384 describes a single photon emitter having a plurality of quantum dots located in a cavity. The dots have a distribution of sizes and hence emission frequencies centred around a central emission frequency with the largest concentration of dots. The cavity is designed to couple to dots with frequencies other than the central emission frequency.
Anders et al, Physical Review B 66 125309 (2002) describes observing a bimodal distribution in the size of InGaAs quantum dots formed at a lattice mismatched interface. An InGaAs layer deposited upon a GaAs substrate forms quantum dots with two distinctly different average sizes and thus produces two distinct peaks in the emission spectrum. The emission wavelength distribution thus consists of two peaks: a first lower wavelength peak; and a second longer wavelength peak.